I am investigating the hypothesis that Hu protein family members have non-redundant functions as negative regulators of the alternative splicing of neurofibromatosis type I (NF1) exon 23a. This research will help to determine the mechanisms behind how these splicing suppressors have diverged in function, and will also shed light on the mechanisms by which Hu proteins regulate splicing, which is not well understood. Preliminary data indicate that HuC is a strong suppressor and HuB a weak suppressor of NF1 exon 23a inclusion. In my project, I will investigate whether HuB and HuC vary in ability to regulate endogenous NF1 splicing in primary mouse cerebellar granule neurons. In addition, I will investigate the mechanism of the difference between HuB and HuC in splicing suppressor potency by performing in vitro splicing and spliceosome assembly assays. I will perform these assays using a novel system involving preparation of nuclear extract from mouse embryonic stem cells differentiated into neurons rather than from immortalized cell lines or whole tissues containing a heterogeneous mixture of cell types. Public Health Relevance: Alternative splicing is a process that is critical for many functions within the nervous system, such as neuronal development, and errors in alternative splicing can lead to disease, making it important to understand how the process is regulated. My study will help to elucidate the mechanism by which the Hu protein family regulates neurofibromatosis type I (NF1) exon 23a inclusion, a splicing event that is important for NF1 function and could possibly affect severity of NF1 disease. In addition, my studies will increase understanding of the molecular functions of the Hu proteins, which are associated with a disease called paraneoplastic syndrome.